The production of products by various endothermic reactions involving steam takes place in a variety of reactors. For instance, synthesis gases (“syngas”) containing hydrogen and carbon monoxide are produced in a reactor known as a steam methane reformer. The steam methane reforming reaction is an endothermic process that involves the reaction of a hydrocarbon containing reactant with steam with a reaction section of the reformer. The endothermic process is driven by heat produced by burning a fuel in the combustion section of the reformer.
Commonly, in steam methane reformers (“SMR's”), the syngas is produced from natural gas. Before entering the SMR, steam is added to natural gas prior to being fed into the reaction section of the SMR. The endothermic reforming reaction is:CH4+H2O← →3H2+CO.
The shift conversion reaction shown below also takes place in the reformer and establishes the equilibrium between the hydrogen and carbon oxide species in the reformed gas:CO+H2O← →H2+CO2.
The fuel used to provide the heat required for the endothermic reaction can also be natural gas. Typically, an air stream and a natural gas stream are fed through burners into the radiant section of the combustion section for combustion of the natural gas supported by oxygen within the air.
There are several approaches that the industry has taken in order to increase the productivity of an SMR. One approach is to increase the firing rate of the primary reformer. The output is increased by burning more fuel, which raises the average temperature on the combustion side of the reforming system. As a result, more heat is transferred to the reaction section and more gas can be processed.
Other approaches employ additional processing equipment. These include the addition of a low temperature shift reactor, a pre-reformer, and a post reformer.
The low temperature shift reactor would follow the high temperature shift unit and convert more of the moisture reacting with carbon monoxide to produce hydrogen. However, it does not increase reformer throughput.
In a pre-reformer, adiabatic steam-hydrocarbon reforming is performed on the process gases prior to introducing the process gases into the reformer. Heat for the reforming reactions is obtained by preheating the feed against hot flue gases in the reformer convection section.
There are two types of post reformers: a bypass-feed product-heat-exchange reformer and an oxygen secondary reformer. The bypass-feed product-heat-exchange reformer uses the heat contained in the reformer product gas to provide the heat to drive additional reforming. The feed to this unit is normally a steam-hydrocarbon mixture that bypasses the primary reformer. The oxygen secondary reformer involves adding oxygen or a steam/oxygen mixture to the output from the primary reformer off-gas and passing the combined mixture through a catalyst bed to convert residual methane to hydrogen and carbon monoxide. Normally, the primary reformer is operated at a higher throughput (greater process gas flow without increasing firing rate). Such an arrangement increases the overall system capacity and provides more methane for conversion in the secondary oxygen unit.
A number of literature references have discussed this subject matter. U.S. Pat. No. 6,217,681 B1 discloses the use of an oxygen rich vent stream as the oxygen source for oxy-fuel combustion or enrichment oxygen in air-fuel combustion to provide heating for primary melting of glass or aluminum. However, there is no teaching or suggestion for the use of the waste oxygen stream in the SMR combustors to enhance hydrogen production.
U.S. Pat. No. 6,200,128 B1 discloses the recovery of heat from a gas turbine exhaust by introducing the exhaust into a combustion device and adding an oxidant having a concentration greater than 21% to form a mixture that has an oxygen content less than 21%. Further, the patent discloses operating the combustion device at conditions substantially equal to those achieved with air combustion of fuel in the combustion device.
Wei Pan et al. (“CO2 Reforming and Steam Reforming of Methane at Elevated Pressures: A Computational Thermodynamic Study” Proc.—Annu Int. Pittsburgh Coal Conference, Vol. 16, 1999, pp. 1649–1695) discloses carbon dioxide reforming and the replacement of steam with oxygen in the carbon dioxide reforming process. The calculations therein provide the equilibrium conditions at given input temperatures and pressures. Steam methane reforming is not specifically discussed and no teaching or suggestion as to how this would be implemented.
V. R. Choudhary et al. (“Simultaneous Steam and CO2 Reforming of Methane to Syngas over NiO/MgO/SA-5205 in the Presence and Absence of Oxygen,” Applied Catalysis A: General, 168, (1998), pp. 33–46) discloses the performance of different gas mixtures on methane conversion to syngas based on a ˜1 ms residence time catalytic reactor. Because of the short residence time, the reaction section is essentially adiabatic, no significant amount of heat transfer is possible. There is no teaching or suggestion for applying catalyst in conventional furnace based reformer systems.
G. J. Tjatjopoulos et. al. (“Feasibility Analysis of Ternary Feed Mixture of Methane with Oxygen, Steam, and Carbon Dioxide for the Production of Methanol Synthesis Gas,” Industrial and Engineering Chemistry Research, Vol. 37, No. 4, 1998–04, pp. 1410–1421) discloses the impact of various mixtures on the thermodynamic equilibrium achieved at the end of the reactor. This reference discloses implementing systems with CH4/O2/H2O mixtures involves a two stage process involving primary and secondary reformers if the ternary mixture is endothermic and a single stage adiabatic unit if the mixture is exothermic.
U.S. Pat. No. 5,752,995 discloses the use of a specific catalyst in reforming reactions including space velocity considerations as well as steam to carbon ratio specifications and the use of oxygen containing gas from a group consisting of steam, air, oxygen, oxides of carbon and mixtures thereof. There is no teaching or suggestion on the addition of oxygen to SMR process feeds to increase the productivity of existing reformers.
EP1 077 198 A2 and EP1 077 198 A3 disclose the addition of a pre-reformer to remove oxygen from the feed to the primary reformer. There is no teaching or suggestion for the addition of oxygen to the primary reformer process feed gas.
Lambert, J. et. al. (“Thermodynamic Efficiency of Steam Methane Reforming with Oxygen Enriched Combustion,” The 5th World Congress of Chemical Engineering: Technologies Critical to the Changing World. Volume III: Emerging Energy Technologies, Clean Technologies, Remediation, and Emission Control; Fuels and Petrochemicals. Jul. 14–18, 1996, San Diego, Calif., Publisher; AIChE, NY, N.Y. pp. 39–44) discloses the use of oxygen-enriched air combustion in combination with steam methane reforming and water gas shift reactions. Lambert et al. discloses improved conversion of methane at constant fuel (furnace firing rates) and process feed gas rates. However, there is no teaching or suggestion as to how this would impact existing reformers.
There are disadvantages to each of the prior art production enhancements. For instance, in a production enhancement that involves increasing the firing rate of the combustion section, lower operating efficiencies result because the temperature and flow of the flue gas leaving the furnace is higher than at normal firing rates and, unless the convective heat recovery section is modified, the stack temperature will be higher than under the original operating mode. Moreover, the higher flow rates and temperatures can exceed fuel system control limits, induced draft fan limits, and excess reformer tube wall temperatures. Changes in control systems, and induced draft fans require capital and time to implement.
The main disadvantage of adding a low temperature shift unit is that it is only an option in cases where one does not already exist. It is to be noted that adding such a unit does not actually increase the capacity of the reforming process. These units are difficult to operate and improve operations by increasing the conversion of reformer product to hydrogen. The low temperature shift option requires additional capital, is limited by the residual carbon dioxide content of the gas leaving the high temperature shift unit, and is of little or no value if the syngas produced by the reformer is used for producing chemicals such as methanol or ammonia.
The addition of a pre-reformer is also a capital intensive endeavor because it involves the addition of a catalytic reactor in addition to modifying the convective heat recovery section to provide the heat necessary for driving the reforming reactions. In addition the export stream that would otherwise be produced is lost because the heat from the convective section used to drive the pre-reformer is no longer available to produce steam. The large quantity of catalyst used in the pre-reformer is generally twice as expensive as that for the primary reformer and has a relatively short life. In addition, the quantity of steam available for export is reduced.
Post reforming accomplished by a bypass-feed product-heat-exchange reformer is also capital intensive because it involves the addition of a catalytic reactor downstream of the primary reformer. Maintenance is difficult on this heat-exchanger reactor. In addition, export steam production is lost because the heat in the exhaust of the primary reformer is used to drive additional hydrocarbon conversion to carbon monoxide and hydrogen. This concept was developed to eliminate or reduce export steam production from the reformer.
An oxygen secondary reformer is a refractory-lined reactor with a combustor located at the entrance to the catalyst bed. The secondary reformer is placed downstream of the primary reformer. Oxygen, or a mixture of oxygen and steam is reacted with the primary reformer product to raise the temperature of the mixture up to about 2,200° F. Relatively large quantities of the oxygen and steam are required to accomplish this temperature rise (600° F. to 800° F.). In addition, significant changes to the carbon dioxide removal system may be required because of the higher levels of carbon dioxide produced to raise the inlet temperature to the reformer.
As will be discussed, the present invention provides a process for increasing the amount of production that can be accommodated within an existing reformer or other reactor that can be effectuated without redesign of the reactor or the addition of expensive components and that is inherently more energy efficient than prior art methods.